The relationship of energy expenditure and spontaneous activity to the aphagia of rats with lesions in the lateral hypothalamus.

1. Lesions in the lateral hypothalamus of rats always produced an immediate increase in total energy expenditure. The increase was maintained for 24 hr or longer only in rats that became and remained aphagic. Rats that showed no recovery from the aphagia and were maintained by tube feeding showed a second, larger increase in metabolic rate after about 7 days.2. The increase in total energy expenditure was associated, initially, with almost continuous motor activity. With continued aphagia an abnormally differentiated pattern of activity appeared at the same time as the second increase in metabolic rate, and the compartment of energy attributable to activity remained high (34% as compared with 23% in normal rats). These changes were accompanied by a recovery of an instrumental response for food (lever pressing), of interest in offered foods and of grooming activity.3. Metabolic abnormality not attributable to activity was indicated by increased creatinine excretion of aphagic but not of hypophagic rats.4. Rats that recovered spontaneous feeding after aphagia returned towards a normal differentiated pattern of activity, with bursts of activity separated by periods of rest, but showed residual abnormality in creatinine excretion.5. The intimate association between increase of energy expenditure, abnormality of motor activity and the aphagia produced by lateral hypothalamic lesions is consistent with the hypothesis that motor incoordination or disorganization is a cause of the aphagia.